Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which transmit an ultrasound beam and receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the focused ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
An ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. Resolution of a scan line is a result of the directivity of the associated transmit and receive beam pair.
The output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Operators of abdominal ultrasound imaging systems often use several transmit zones per direction in order to obtain a better focused image. For each scan line, transmit focus is varied so that the image is made up of individual zones with optimized focus. There may be up to M=10-15 transmit focal zones. The resulting images are "stitched" together, giving a composite image.
The size of each zone is proportional to the depth of field EQU L.varies..lambda.(RID).sup.2
where R is the focal depth, D is the aperture and .lambda. is the wavelength. For a constant f/number (=R/D) and wavelength and with M zones, the useful range is EQU L.sub.zones =M.times.L
This composite method provides more uniform image quality and is routinely employed in radiology ultrasound equipment. This method depends on the object to be imaged being stationary during all M image acquisitions, and is also dependent on accurate gain matching between the zones in order for the image to appear as a single image of uniform brightness without bands.
In many situations one would desire the high image quality of this method, but the imaged object is not stationary enough to allow for all the M transmit zones to be acquired without motion artifacts. Since M zones cause the frame rate to be reduced by a factor of almost 1/M compared to using just a single transmit focal zone, the problem is that the achieved image update rate or frame rate is not high enough.
The synthetic aperture focusing technique is the classical synthetic aperture method where, in each instant, only a single element is used for transmission and reception. Simple transmit and receive electronics are needed, but the synthetic aperture focusing technique requires data memory for all N data recordings. Multi-element synthetic aperture focusing is an improvement, which increases acoustic power and signal-to-noise ratio.
Synthetic focusing is a method where, in each instant, one element transmits a pulse and all elements receive echo signals. This approach allows full dynamic focusing during both transmit and receive, which yields the highest image quality. Unlike the synthetic aperture focusing technique, a full data set of N.sup.2 RF-lines is used and needs to be stored. Like the synthetic aperture focusing technique, synthetic focusing may be sensitive to motion artifacts.
A synthetic receive aperture technique has also been proposed to enable an imaging system to address a large number of transducer receive elements without the same number of receive channels.
There is need for a method of focusing that provides the high image quality of the composite method without as much of a reduction in frame rate. This method should require fewer transmits and less RF memory than that required by the synthetic focusing method.